This invention relates to an image formation system such as an electrophotographic copier or a printer and in particular to an improvement in a double-sided image formation system that can form an image on both sides.
Already known as a conventional double-sided image formation system is, for example, a system wherein two photosensitive bodies are placed facing each other and a first toner image (first image) is formed on one photosensitive body and a second toner image (second image) is formed on the other, then the first and second toner images on the photosensitive bodies are transferred to both sides of paper at the same time. (For example, refer to the Unexamined Japanese Patent Application Nos. Sho 63-63057 and Hei 2-259670.)
Also known is a system which comprises one photosensitive body, such as a photosensitive drum, on which a first image and a second image are supported, an intermediate transfer belt for once holding the first image, a first transfer device being disposed in a first transfer part for transferring each image on the photosensitive body to the intermediate transfer belt or paper, and a second transfer device and a paper stripping device being disposed at the paper discharge end of the intermediate transfer belt, the second transfer device for transferring the first image on the intermediate transfer belt to paper. (For example, refer to the Unexamined Japanese Patent Application Publication No. Hei 1-209470.)
Further, the present applicant already proposes an art for primarily transferring a first image on a first photosensitive body to a first intermediate transfer belt through a primary transfer device and a second image on a second photosensitive body to a second intermediate transfer belt through the primary transfer device and transferring the images on the first and second intermediate transfer belts to both sides of paper at the same time by means of a pair of transfer rolls between which the intermediate transfer belts are sandwiched. (For example, refer to the Japanese Patent Application No. Hei 8-108449.)
However, such a double-sided image formation system forms images on both sides of paper in two image formation sections and the image formation conditions in the two image formation sections easily vary, thus a technical problem that a density difference and a color difference easily occur between images on both sides of paper is involved.
Particularly, in a form wherein images are transferred to both sides of paper at the same time, a danger that the charge amount of a developed image on the polarity inversion side may differ from that on the polarity non-inversion side is high although only one transfer electric field can be set, thus a density difference and a color difference more noticeably easily occur between images on both sides of paper.
By the way, a technique of developing a density sensing patch on a photosensitive body, applying light to the patch, and reading its reflected light or diffused light by a sensor for keeping track of the development amount is known as a density sensing technique in a general electrophotographic image formation system. A known image formation system using an intermediate transfer body reads a patch density after primary transfer on the intermediate transfer body by a sensor and a patch density after secondary transfer to a secondary transfer roll by a sensor (the Unexamined Japanese Patent Application Publication No. Hei 8-44122).
However, the density sensing sensors in such related arts are provided only for keeping track of the image density in one image formation section; they are not designed for use with two image formation sections and are not indented for use so as to eliminate the density difference between two image formation sections.
Therefore, the density sensing techniques in the related arts cannot be used directly as sensors for keeping track of the density and color information of images on both sides of paper to eliminate the image quality difference (density difference and color difference) between the images on both sides of paper in a double-sided image formation system.
As disclosed in the Unexamined Japanese Patent Application Publication No. Hei 8-44122, the system sensing a patch density after secondary transfer onto a secondary transfer roll by a sensor may enable an image density closer to an actual image on paper to be sensed with high accuracy.
However, the secondary transfer roll, which is not originally an image support member, is poor in surface property as compared with an image support member such as a photosensitive body or an intermediate transfer belt; there is apprehension that if a patch image is prepared on the secondary transfer roll surface, it becomes easily hard to sense the patch density with good accuracy and there is also apprehension that when a patch image is prepared on the secondary transfer roll surface, a cleaner dedicated to removing of the patch image must always be provided and the system configuration becomes complicated accordingly.
To prepare patch images on the secondary transfer roll, the patch image size and the number of patch images prepared cannot exceed the outer periphery of the secondary transfer roll at a time and moreover patch images of reasonable size are required from the viewpoint of facilitating density sensing, thus the flexibility of preparing patch images is restricted naturally.
Particularly, when density adjustment is made for four colors in a color image formation system, a patch image must be prepared for each color on a secondary transfer roll.
At this time, to prepare patch images continuously, sequential cleaning of the secondary transfer roll after batch image formation becomes required; a normal secondary transfer roll, which is formed of a foamed substance, etc., is hard to clean and it takes time until cleaning ends. A technical problem occurs that the density adjustment time of each color in the color image formation system increases unnecessarily.
Moreover, to prepare patch images continuously, a secondary transfer roll needs to be of size for preparing 4-color patch images, but a normal secondary transfer roll is about .PHI.12 mm, for example, and the size is insufficient to prepare 4-color patch images.